Dimension measurement

INDUSTRIAL INSTRUMENTATION

Presentation on

DIMENSION MEASUREMENT

Presented to:

Dr. –Ing. Naveed Ramzan

Presented by:

Noor Ul Ain Sabir

2008-CHEM-09

Contents 1. Dimension measurement

• Why to measure dimensions

2. Thickness measurement

• Thickness gauges

3. Length measurement

• Laser based length measurement

4. Width measurement

• Camera based width measurement

5. Diameter measurement

• Laser based diameter measurement

REFERNECES 3

1. Dimension measurement

The determination of linear and angular magnitudes of particular

features or mechanical parts of a prototype or machine itself is

called as Dimension Measurements.

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Why to measure dimensions???

Dimension measurement and regulations are among the quality

control parameters of industrial machinery design. This practice is

done to

• Meet the customer’s requirements

• Avoid any deviation in dimensional tolerances

• Avoid rejection of the product by the customers

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Dimensional requirements

The following parameters of an object are normally measured or

determined in routine:

• Thickness

• Length

• Width

• Diameter

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2. Thickness measurement

In some industrial productions the control or measurement of

thickness of coating, applied to a base material is essential e.g.

• Deposition of zinc on steel

• Application of wax, clay bitumen or plastic on paper etc.

This base or sheet material thickness measurement with and

without coating material is done with the help of a device known

as ‘thickness gauge’.

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Thickness gauges

Contact type gauges

Inductive type

Capacitive type

Non-contact type gauges

Radiation type

Laser based

Coating gauges Differential beta

gauge

Beta backscatter gauge

Thickness gauges a. Contact type thickness gauges

i. Inductive type

• Reluctance transducer

• Eddy current transducer

ii. Capacitive type

b. Non-contact type thickness gauges

i. Radiation type

ii. Laser based

c. Coating thickness gauges

i. Differential beta thickness gauge

ii. Beta backscatter thickness gauge

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a. Contact type thickness gauges

Here we find that

• Thickness of the object is found through electrical methods.

• The gauges make measurements upon a physical contact with

the test piece

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i. Inductive type thickness gauge

Two kinds of transducers are utilized:

• Reluctance variation transducers: These devices measure change

in inductance of a magnetic coil caused by the test object.

• Eddy current transducers: They use eddy currents to find the

thickness of test object.

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Reluctance variation transducer

• It consists of a U-shaped magnetic core over which a coil is

wound .The stock or a test piece, whose thickness is to be

measured, completes the magnetic circuit.

• Inductance of the coil depends upon the reluctance of the

magnetic circuit, which in turn depends upon the thickness of

the test object

Reluctance transducer

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Reluctance transducer

Reluctance transducer

• The non-magnetic test piece is placed on ferromagnetic base; the

base is thick such that the reluctance of the magnetic circuit is

determined by the distance between the ends of the U-core and

the base plate.

• An increase in thickness of the test piece causes an increase in

reluctance and a decrease in inductance of the magnetic circuit.

Eddy current transducer* (Conducting material)

• It has a coil wounded on an insulated core, excited by an

alternating current supply. The alternating field produced as a

result generates eddy current in the test piece.

• The opposition created by the magnetic field of eddy current

against the magnetic field of coil, reduces the inductance of the

coil.

• So, higher the thickness of the test piece, higher will be the eddy

current, lower would be the inductance of the coil.

Eddy current transducer

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Eddy current transducer (Non-Conducting)

• For non-magnetic, non-conducting materials thickness

measurement is done by depositing it on a metal backing.

• If the thickness of the test piece is large, the eddy current

transducer head and the metal backing are separated by a larger

distance and, therefore, the eddy currents are small and

consequently the inductance of the coil is large.

Eddy current transducer

Inductive type gauges

• Advantages

- Measurement of thickness of aluminum sheet 125-500 pm

- Easy to align

- Excellent repeatability

• Disadvantages

- Physical contact causes error in measurement

- It shows temperature sensitivity

- Inherently low power output

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ii. Capacitive type thickness gauge

• This method is used for the thickness measurement of insulators.

C = capacitance in farad

Ɛ0= 8.85 x 10-12 farad/m2

Ɛr = dielectric constant (relative permittivity)

A= area of each plate (m2)

d= distance between the plates (m)

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Capacitive thickness measurement

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Capacitive type gauges • Advantages

- Their frequency response is good.

- They require little force for operation.

• Disadvantages

- They are sensitive to temperature changes,

- They become contaminated with dust particles

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b. Non-contact thickness gauges

They measure the thickness without physical contact and are of

following three types

i. Radiation type

ii. Laser based

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i. Radiation type non-contact gauge

• These gauges employ alpha, beta, gamma or X-ray radiation

obtained from a radioactive element for thickness measurement.

• The radiation source is shielded all around except in the direction

of the detector.

• The radiation source radiates to the detector through the test

object.

• The radiation received at detector are amplified and calibrated to

find the thickness of object.

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Radiation type non-contact gauge

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Radiation type non-contact gauge

• Advantages

- It gives the thickness measurement of sheets in rapid motion without physical

contact.

• Disadvantages

- These are subjected to error because of radioactive decay of source and

dependence of absorption coefficient on the composition of material

- Frequent calibration checks are required for better accuracy.

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ii. Laser based non-contact gauge

• These gauges employ a laser beam.

• The beam is directed at a rotating (or oscillating) mirror to

collimate in to a straight ribbon.

• As the collimated beam scans across an object, it blocks the light

detector on the other side and measures the time for which the

light was blocked. Thus, scan time across the shadow is used to

measure the thickness of an object.

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Laser based non-contact gauge*

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c. Coating thickness gauges

To measure the thickness of coating materials, two types of gauges

are used

i. Differential beta gauge

ii. Capacitive sensor for measuring coating thickness

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i. Differential beta gauge

This technique involves two beta gauges.

• The readings are obtained by passing the un-coated sheet through

one measuring head and the coated from the other. The intensity

of radiation emitted is a measure of total thickness.

• They found application in the measurement of thickness of wax

and plastic coatings applied to paper and aluminium sheet when

the weight of coating is 25% or more of the weight of substrate.

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Differential beta gauge*

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3. Length measurement

Laser Doppler Velocimeters (LDVS)

• The laser Doppler velocimeter sends a monochromatic laser beam

towards the target and collects the reflected radiation.

• According to the Doppler Effect, the change in wavelength of the

reflected radiation is a function of the targeted object's relative

velocity.

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Laser Doppler Velocimeter

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4. Width measurement

Camera based width measurement is an online non contact

technique, using two configurations.

• Stereoscopic vision based:

It is used where the strip thickness changes substantially.

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Camera-based width measurement

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5. Diameter measurement

• It is a laser based system to measure diameter of any type of

opaque rod or rounds.

• The laser beam emitted from the laser diode in the transmitter is

converged into a parallel beam by the projecting lens unit.

• The laser beam is then directed through the slit on the receiver

and focused on the light receiving element.

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Laser Diameter gauge*

• Construction and working

- The gauge consists of two diameter measuring sensors which are

kept such a way to measure axes of the rod at tow angles of

interest.

- The laser transmitter-receiver pairs are kept facing each other at

some distance. When a test rod is passed through the

measurement area, the light falling on the receiver will reduce, in

the amount of diameter of the rod.

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Laser diameter gauge

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Laser diameter gauge

Advantages

• Non-contact type

• Online measurement of diameter

• Versatile and provide accurate measurements

Summary

• Dimension measurement is required to assure top quality of

products as per customer specifications.

• Thickness gauges can be of Contact and Non-contact type and use

inductance, capacitance, laser technology and beta radiations for

measuring the thickness of base materials and applied coating

surfaces.

• The length of an object can be found by subjecting it to a laser

source and thus noticing the shift in frequency of laser beam.

Summary

• The width of object can be found by employing Stereoscopic vision

principle.

• The diameter of an object is found using laser transmitter and

receiver arrangement, the decrease in intensity of radiations

received is a direct measure of diameter.

Questions

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REFERENCES

• Singh. S.K. 2009. Industrial instrumentation and control, 3rd Edition,

McGraw Hill publishing company limited, New Delhi; 225-238

• www.ldvsysteme.de

• www.weldedtubepros.com

• http://www.youtube.com/watch?v=zJ23gmS3KHY

• http://www.youtube.com/watch?v=Ucf2TAKPbPA

• http://www.youtube.com/user/laplaser

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